The important factors that determine the ability of polymer coatings to protect structural aluminum surfaces from corrosion are the magnitude of the wettability of the Al surface by liquid polymer materials and the stability of the interaction products formed at polymer-to-Al interfaces. In order to achieve enhanced wettability, the Al surface should have a high surface free energy for enhanced surface reactivity and sufficient roughness to provide a large surface area for promoting wetting and mechanical locking. If the chemical interaction at the polymer/Al joint results in the formation of valence bonds, mainly covalent, the interfacially-formed interaction products will not only result in an increase in the basic adhesion, but also contribute to a modification of the chemical composition at the interfacial regions. This modification should be associated with the formation of hydrophobic interaction products which can be expressed as passivating layers.
To date, two commercial surface preparations for Al, the Forest Products Laboratory (FPL) preparation [Eichner et al., Forest Products Laboratory Report No. 1813, Madison, Wis., 1950] and the Phosphoric Acid Anodization (PAA) process [Kabayaski et al., Boeing Corporation Report No. D6-41517, Seattle, Wash., 1974], have been widely applied to promote interfacial bond strength at aluminum adhesive joints. The purpose of these surface treatments is not only to increase the roughness of the Al surface thereby enhancing the mechanical interlocking bonds, but also to modify the surface chemical compositions.
One significant problem that has been encountered with these commercial surface preparations is that when the freshly etched aluminum surface is exposed to moisture, hydration begins to occur. Considerable attention has been given to the growth and transformation of the FPL oxide to a hydrated oxide Al in the interfacial regions as this interface was exposed at various times to a humid environment. The chemical transformation commences when the moisture penetrates through the polymer layer and reaches the original adherend oxide adjacent to the adhesives. The reaction of Al oxide with moisture results in the formation of the hydrated oxide Al which represents a different morphology from the original oxide. This interfacial conversion of Al oxide to hydroxide leads to the generation of adhesion stress and swelling and the promotion of crack propagation at or near the Al-hydroxide interfaces, thereby resulting in bond failure and the initiation of corrosion. In either the FPL or PAA treatment, an oxide adherend that will resist attack by moisture is the critical element for bond durability.
Prior art approaches to dealing with the moisture problem involve the tailoring of the reactive surface nature using organosilane and titanate derived coupling agents as chemical modifications for Al oxide and hydroxide surfaces. However, oligomers or unreacted mono silanols are still present in the coupling layers. The presence of unreacted functional silanol leads to the hydrolysis of the coupling layer brought about by penetration of moisture through the adhesive, as the adhesive/coupling/adherend joint system is exposed to high humidity environments. This relates directly to the hydrolytic delamination failure mode.
The simplicity of polyacid molecules such as the polyacrylic acid (PAA) and polyitaconic acid (PIA) macromolecules, which consist of --CH.sub.2 --CH-- main chains and functional carboxylic acid pendent groups, makes them very attractive for use in resolving the problems presented by the prior art materials. Work related to the nature of interfacial reactions which play key roles in determining the extent of bonding between PAA/PIA and crystalline hydrate conversion coatings deposited as corrosion protective films on steel surfaces is known [Sugama et al., J. Mater. Sci., 19, 4045, 1984]. Even though the rough surface morphology of the conversion coatings enhances the interfacial mechanical bonding, the regularly oriented pendent carboxylic acid groups at the interface are readily accessible to proton donor-acceptor interactions to form hydrogen bonds with the polar hydroxyl groups which occupy the outermost surface sites of hydrated crystal layers. This interaction behavior of PAA/PIA has been found to play an essential role in promoting good interfacial bond performance.